feat: per-user auto-memory with ResponseContext

Three memory channels: hidden tool (sol.memory.set/get in scripts),
pre-response injection (relevant memories loaded into system prompt),
and post-response extraction (ministral-3b extracts facts after each
response). User isolation enforced at Rust level — user_id derived
from Matrix sender, never from script arguments.

New modules: context (ResponseContext), memory (schema, store, extractor).
ResponseContext threaded through responder → tools → script runtime.
OpenSearch index sol_user_memory created on startup alongside archive.

src/archive/schema.rs

@@ -24,6 +24,15 @@ pub struct ArchiveDocument {
     pub edited: bool,
     #[serde(default)]
     pub redacted: bool,
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub reactions: Vec<Reaction>,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct Reaction {
+    pub sender: String,
+    pub emoji: String,
+    pub timestamp: i64,
 }
 
 const INDEX_MAPPING: &str = r#"{
@@ -45,7 +54,15 @@ const INDEX_MAPPING: &str = r#"{
             "media_urls":  { "type": "keyword" },
             "event_type":  { "type": "keyword" },
             "edited":      { "type": "boolean" },
-            "redacted":    { "type": "boolean" }
+            "redacted":    { "type": "boolean" },
+            "reactions":   {
+                "type": "nested",
+                "properties": {
+                    "sender": { "type": "keyword" },
+                    "emoji":  { "type": "keyword" },
+                    "timestamp": { "type": "date", "format": "epoch_millis" }
+                }
+            }
         }
     }
 }"#;
@@ -63,6 +80,25 @@ pub async fn create_index_if_not_exists(client: &OpenSearch, index: &str) -> any
 
     if exists.status_code().is_success() {
         info!(index, "OpenSearch index already exists");
+        // Ensure reactions field exists (added after initial schema)
+        let reactions_mapping = serde_json::json!({
+            "properties": {
+                "reactions": {
+                    "type": "nested",
+                    "properties": {
+                        "sender": { "type": "keyword" },
+                        "emoji":  { "type": "keyword" },
+                        "timestamp": { "type": "date", "format": "epoch_millis" }
+                    }
+                }
+            }
+        });
+        let _ = client
+            .indices()
+            .put_mapping(opensearch::indices::IndicesPutMappingParts::Index(&[index]))
+            .body(reactions_mapping)
+            .send()
+            .await;
         return Ok(());
     }
 
@@ -102,6 +138,7 @@ mod tests {
             event_type: "m.room.message".to_string(),
             edited: false,
             redacted: false,
+            reactions: vec![],
         }
     }